In producing a semiconductor element, etc., there is used a projection aligner of stepper type or step and scan type, which can transfer a reticle (a photomask) pattern onto the to-be-irradiated areas of a wafer having thereon a photoresist film (hereinafter referred to as “photoresist” in some cases) formed by coating, via a projection optical system. The projection optical system of projection aligner gives a higher resolution as the wavelength of the exposure ray used is shorter and the numerical aperture of the projection optical system is larger. Therefore, as the integrated circuit has become finer, the wavelength of exposure ray (which is the wavelength of the radiation used in projection aligner) has become shorter year by year and the numerical aperture of the projection optical system has become larger.
In conducting exposure, not only the resolution of projection optical system but also its depth of focus are important. The resolution R and the depth of focus δ are represented by the following formulas, respectively. In order to obtain a certain resolution R, use of a radiation of shorter wavelength can give a larger depth of focus δ.R=k1·λ/NA  (i)δ=k2·λ/NA2  (ii)(In the above formulas, λ is a wavelength of exposure ray; NA is a numerical aperture of projection optical system; and k1 and k2 are each a process coefficient.)
A photoresist film is formed on the wafer to be irradiated and a pattern is transferred onto this photoresist film. In conventional projection aligners, the space in which the wafer is placed, is filled with air or nitrogen. When the space between the wafer and the lens of projection aligner is filled with a medium of refractive index n, the above-shown resolution R and depth of focus δ are represented by the following formulas.R=k1·(λ/n)/NA  (iii)δ=k2·nλ/NA2  (iv)
For example, in an ArF process, when water is used as the above medium and there is used n=1.44 (the refractive index of water in a ray of wavelength of 193 nm), the resolution R becomes 69.4% [R=k1·(λ/1.44)/NA] and the depth of focus becomes 144% (δ=k2-1.44λ/NA2), as compared with the exposure case of using air or nitrogen as a medium.
Such a projection exposure method in which the wavelength of a radiation to be applied is made shorter and a finer pattern can be transferred, is called liquid immersion exposure; and this liquid immersion exposure is considered to be an essential technique in conducting finer lithography, particularly lithography of several tens nm unit, and a projection aligner therefor is known (see Patent Document 1).
In the method of liquid immersion exposure using water as the medium, both the photoresist film formed on a wafer by coating and the lens of projection aligner come into contact with water. As a result, water penetrates into the photoresist film, which may lower the resolution of the photoresist. Further, components constituting the photoresist dissolve out into the water, which may stain the surface of the lens of the projection aligner.
In order to prevent the contact between the photoresist film and the medium (e.g. water), there is a method of forming an upper layer film (a protective film) on the photoresist film. However, this upper layer film is required to have properties such as mentioned below.    (1) It has sufficient transmission for the wavelength of the radiation used.    (2) It can be formed on a photoresist film while causing substantially no intermixing with the photoresist film.    (3) In liquid immersion exposure, it does not dissolve in the medium (e.g. water) used and can maintain a stable film state.    (4) It is easily soluble in a developing solution (e.g. an alkali solution).As conventional relevant technical documents disclosed, there are Patent Document 2, Patent Document 3, etc.
Patent Document 1: JP-A-1999-176727
Patent Document 2: JP-A-2005-264131
Patent Document 3: JP-A-2006-64711